docs(ngModelController): update setValidity
docs(ngModelController): update setValidity

Needs to be `$error[validationErrorKey]!=isValid` and not
`$error[validationErrorKey]=isValid`.

See https://github.com/angular/angular.js/blob/master/src/ng/directive/input.js#L1627

Closes #7934

ggershoni/angular.js,ggershoni/angular.js,ggershoni/angular.js,ggershoni/angular.js